A gear transmission is known in which, while an external gear is meshing with an internal gear, one of the gears rotates eccentrically relative to the other of the gears. Such a gear transmission may be called an eccentric oscillating-type. In the eccentric oscillating-type gear transmission, the number of teeth of the internal gear and the number of teeth of the external gear differ. In case the external gear is an eccentrically rotating type, the external gear is supported by a carrier. When the external gear rotates eccentrically while meshing with the internal gear, the carrier rotates slower relative to the internal gear in accordance with the difference in the number of teeth of the two gears. An example of this type of gear transmission is disclosed in Japanese Patent Publication No. H5-180278. In the description below, Japanese Patent Publication No. H5-180278 is called Patent Document 1. As illustrated in Patent Document 1, the internal gear often includes a plurality of grooves formed along a circumferential direction on an inner circumferential surface of a case, and cylindrical members inserted into the grooves. Moreover, in the type of gear transmission in which the internal gear rotates eccentrically, the internal gear is supported by a carrier. When the internal gear rotates eccentrically, the carrier rotates slower relative to the internal gear.